Stephen Hawking wins special physics prize

Special Fundamental Physics Prizes have been awarded to a team of seven scientists who led the efforts at CERN’s Large Hadron Collider (LHC) and, separately, to Stephen Hawking for his discovery that black holes radiate.


These two awards highlight how strong a role British physicists play on the world stage. From the three British-based physicists leading efforts at LHC – Lyn Evans, Michel Della Negra and Tejinder Singh Virdee – to Stephen Hawking, we should rightly cherish our nation’s strength in physics.

Professor Sir Peter Knight, President of the Institute of Physics.

Black holes are not black!
In 1975 Hawking published a paper with a rather surprising result: if one takes into account quantum theory black holes are not quite black. Rather, a black hole glows slightly due to “Hawking radiation”, which consists largely of photons and to a lesser extent other particles.

The corresponding temperature is

\(T \approx \frac{1.227 \times 10^{23} }{M}\) Kelvin, and here the mass is in KG.

Because of this, the temperature would only be significant for very light black holes. For example, a 30 solar mass black hole has a temperature of about \(2\times 10^{{-}9}\) Kelvin and the corresponding luminosity is about \(10^{{-}31}\) Watts. Bigger black holes would hardly radiate at all. This would be completely swamped by other sources of radiation including the CMBR.

Evaporation of a micro black hole
Black holes get the energy for the Hawking radiation from their rest mass. So, assuming nothing is falling into the black hole, a radiating black hole will be loosing mass. For astrophysical black holes this loss will be miniscule. The evaporation time for a black hole of 30 solar masses is about \(10^{60}\) times the age of the Universe!

For much much smaller black holes Hawking radiation could lead to compete evaporation. For example, a black hole with the mass of about a mountain could evaporate in a time scale that is less than the age of the Universe.

However, no-one is really sure what happens to a black hole near the end of its evaporation.

The black hole information paradox
Classically, there are only three things we can know about a black hole; its mass, its angular momentum and its electric charge. These are the only parameters that describe a classical black hole. I paraphrase this as “black holes do not care what they eat”.

No matter what properties the matter that falls into a black hole has, say baryon number or lepton number, the black hole classically only cares about the mass, angular momentum and electric charge. All the other information is hidden inside the black hole away from the rest of the Universe.

So now suppose our black hole evaporates and disappears. An important property of Hawking radiation is that it is thermal: that is completely random. The information content of a black hole (what it has eaten!) appears to be lost when it dissipates.

What happens to this information is a far from understood question in physics.

Hawking’s work on black hole radiation has posed more questions about nature than it has really answered. That is the true sign of great work.

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